JK Rowling brought out her first Harry Potter book a year before that and in an initial print run of only 1,000 copies. It was some time before her stories became best-sellers.

Fifty Shades of Grey sold 14,814 copies in its first week, although it went on to become the best-selling debut of all time. Dan Brown’s Angels and Demons sold only 98 copies in its first week of UK release, and Stephenie Meyer’s Twilight just 53 copies.

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While those titles built over time through word-of-mouth, Girl Online had the benefit of Zoella’s pre-existing fanbase. She has six million YouTube subscribers, and drew on elements of her own life to write the tale of a schoolgirl blogger who finds love on a trip to New York.

She landed a two-book deal with Penguin Random House earlier this year. The CEO, Tom Weldon, said he was initially doubtful about the wisdom of signing a YouTube star but did so after consulting his 13-year-old goddaughter.

“She said, ‘Oh my God, you should buy Zoe’s first novel, she is amazing, the book will be a huge best-seller,” Weldon said at Sugg’s book launch last month.

He predicted then that Girl Online would be “absolutely enormous” and said Penguin Random House would be her publisher “for many, many years to come”.

Reacting to news of her record-breaking fiction debut, Sugg said: ‘It’s such an amazing feeling. I’m so grateful to everyone who has bought a copy of Girl Online. I love that so many of my viewers are enjoying the book! This year has been so exciting and this for sure is the icing on the cake."